Add memory gating to query2

[d-callan]

• Add --memory-budget CLI param (MB, 0=no limit)
• Implement memory_semaphore with RAII sem_guard for exception-safe gating
• Estimate per-sub-index memory as heap peak + mmap working set:
  • Heap: max(phase1=smers+responses, phase2=agg with positions/ratios/counts)
  • Mmap: bloom_size (no --fast) or index_size (--fast, all partitions resident)
  • Compressed indexes: bloom_size * nb_partitions with --fast (decompressed to heap)
• Sort sub-indexes by descending memory
• Gate task execution via semaphore acquire/release
• Warn when an index exceeds budget (runs alone)
• Add 5 tests: baseline, tight budget, --fast+budget, position format+budget, oversized index warning